What is sandwich structure????? • Thin composite skins bonded to thicker, lightweight core. • possesses sufficient stiffness in the direction normal to the plane of the faceplates History…… • Branches of tree ,Bones etc.- Foam-like core materials • Subjected to complex loadings • W. Fairbairn(1849) -first person -to describe the sandwich construction principle -Britannia Tubular Bridge in North Wales. • Iron compression sheets riveted to both sides of a wood core. • In 1924, T. von Karman and P. Stock patented a glider plane applying sandwich for fuselage structure. • Apollo project, that successfullylanded on the moon in 1969. • A310 was the first aircraft in the AIRBUS fleet to be equipped with a composite honeycomb sandwich rudder. Why better than other materials ??? • It is an extremely lightweight structural approach that exhibits high stiffness and strength-to-weight ratios. • much like the I-beam comparison Efficiency of Sandwich Structure Advantages • Very high stiffness-to-weight and high bending strength-to-weight ratio • Enhances the flexural rigidity of the structure without adding substantial weight. • Fatigue strength • Insulation • Thermal insulation. Face Materials Any structural material which is available in the form of thin sheet can be used Requirements: High stiffness High tensile and compressive strength Good impact resistance Good surface finish Environmental resistance Wear resistance Typical materials used : Manufacturing 1. Vacuum bagging and autoclaving 1. Used for more advanced structures, e.g. for competition yachts and in aerospace applications 2. Both monolithic laminates and sandwich structures are usually laid up using prepregs. 3. Manufacturing of sandwich components with faces made from prepregs may be accomplished in two overall manners. 4. Under certain conditions, the laminate may be laid directly onto the core in a fashion similar to that used in wet hand lay-up. 5. Alternatively, previous manufactured single-skin laminates may be adhesively bonded to the core in a separate process. • technique involves placing and sealing a flexible bag over a composite lay-up, evacuating all the air from under the bag. The removal of air forces the bag down onto the lay-up with a consolidation pressure of up to 1 atm. • Laid-up prepreg stack is prepared as shown in the Fig • The completed assembly, with vacuum still applied, is placed inside an oven or on a heated mold, and the composite is produced after applying an appropriate temperature curing cycle. • Used for the manufacture of high-quality structural component. • Similar to vacuum bag technique but oven is replaced by autoclave • It allows the application of atmospheric pressure and provides the curing conditions for the composite where the application of vacuum, pressure, heat-up rate and cure temperature are controlled. • For autoclave processing, sandwich constructions can usually be laid up and cured as a single-shot process. But, the use of excessive pressures can led to eventual core crush. 2.Adhesive Bonding • Simple manufacturing operation, independent of face and core materials ,structure made by this said to have good mechanical properties • Adhesive layers are interleaved between the faces and the core, and the whole stack is subjected to a temperature and pressure cycle, depending on the adhesive resin used. • The adhesives can be used in film or liquid form, depending on the application, and are usually epoxy or polyurethane. • Before bonding operation, it is necessary to prepare the surface using suitable techniques like degrading, degreasing or chemical treatment. After preparation, the core is carefully placed between the faces after applying the adhesive films . • After processing ,using an autoclave or vacuum bagging apply the required vacuum and temperature 3. Wet lay-up • Oldest and one of the most flexible technologies • Can be done by hand lay-up or spray lay-up • A gel coat is deposited directly onto the laminate to have good environmental resistance and producing a smooth, cosmetically appealing surface and the reinforcements in the form of mats are impregnated by hand with resin, with the help of brush or roller. • In the case of spray-up, fiber is chopped in a handheld gun and fed into a spray of catalyzed resin directed at the mould. • Laminates are cured at atmospheric conditions or with the help of vacuum bagging. 4. Liquid Molding • The reinforcement is first placed in the mold whereupon the liquid resin is infused into the reinforcement fabric through the difference in pressure. • The liquid molding processes used include: 1. Resin Transfer Molding (RTM) 2. Structural Reaction Injection Molding (SRIM) 3. Vacuum-Injection Molding • Received much interest in recent years due to its capability of producing geometrically complex structures in an economical fashion, without creating an unhealthy work environment since the processes are closed molds. • In RTM, the reinforcement fabrics or mats, core materials and eventual inserts are conveniently placed in the mould that is then closed. • Afterwards, the resin system is pressurized, using an injection machine that pumps and mixes in the right proportions the resin and catalysts, until full impregnation is obtained. • The resin is often heated to lower the viscosity and thus facilitate impregnation. The resin infusion is stopped when the resin font has reached all the ventilation holes in the mold and the resin starts to leak out. • The processes of RTM, SRIM, and vacuuminjection molding are distinguished from one another primarily by the type of resins, molds, and impregnation techniques. • In vacuum-injection molding, conventional preformulated resins, similar to those used in the wet lay-up processes, are used. In contrast, SRIM employs highly reactive resins that are mixed right before injection. • In RTM and SRIM rigid, matched molds are used, whereas vacuum-injection molding employs a one-sided mold, often a marginally modified version of wet lay-up mold, covered by a vacuum bag. • In RTM and SRIM ,the resin are injected using a pump but in vacuum moulding effect of vacuum is used to pull in the resin. • The resin filling time and demoulding of parts in SRIM are much faster compared other two process.
